ALARA Stay-Time Arithmetic and Review Boundaries

Stay Time Calculations

The basic arithmetic prompt is: time prompt = dose-budget prompt / dose-rate prompt. If the dose-rate row is 50 mR/hr and a task dose-budget prompt is 100 mrem, the local arithmetic returns 100/50 = 2 hours. That value is a review input, not entry authorization.

The dose-budget prompt should come from the current radiation protection program, RSO procedure, ALARA review, or work planning process. The annual occupational TEDE source row under 10 CFR 20.1201 is 5 rem (50 mSv), but annual regulatory rows are source context and should not be treated as task targets by themselves.

For a specific job, the dose budget depends on expected dose rate, task duration, worker dose records, monitoring period, administrative controls, and procedure requirements. If a worker has an official 300 mrem value for the relevant period and a local 500 mrem administrative prompt, the local remaining prompt is 200 mrem; at 100 mrem/hr, the arithmetic prompt is 2 hours.

When dose rate varies across the work area, calculate dose for each segment separately and add the segment doses. If a worker spends 30 minutes in a 20 mR/hr zone and 15 minutes in a 100 mR/hr zone, the total arithmetic is (0.5 × 20) + (0.25 × 100) = 35 mrem. The procedure still needs survey, work-position, dosimetry, and RSO review.

10 CFR 20 Dose Limits

The NRC dose-limit rows in 10 CFR 20.1201 are regulatory source context. They are legal ceilings, not task targets, and they do not by themselves establish a worker dose budget for a job.

• Total Effective Dose Equivalent (TEDE): 5 rem (50 mSv) per year for adult occupational whole-body context.
• Organ or tissue dose equivalent (excluding lens of eye): 50 rem (500 mSv) per year.
• Lens of eye dose equivalent: 15 rem (150 mSv) per year.
• Shallow dose equivalent (skin or extremity): 50 rem (500 mSv) per year.
• Embryo/fetus of declared pregnant worker: 0.5 rem (5 mSv) over the entire pregnancy, with an effort to keep dose relatively uniform over time.

For individual members of the public, 10 CFR 20.1301 includes 100 mrem (1 mSv) per year and 2 mrem in any one hour in an unrestricted area. That row is public-dose context, not a stay-time authorization or complete radiography boundary analysis.

Use these rows as source pointers. Actual dose assignment, monitoring, reporting, and corrective-action decisions depend on official dose records, survey records, license conditions, procedures, and qualified review.

ALARA Planning for Routine and Non-Routine Work

ALARA is the practice of keeping radiation exposures as low as reasonably achievable, considering economic and practical factors, within the radiation protection program. A stay-time calculator can support the arithmetic, but it cannot complete the review.

Routine work may be covered by standing procedures that define surveys, dosimetry, area controls, and review triggers. Dose trends and action levels stay in the program records.

Non-routine work usually needs a documented review before work starts. That review may include survey rows or estimates, expected task durations, calculated dose prompts, dose-reduction measures, administrative dose-budget prompts, communications, supervision, and contingency actions.

Three common strategies reduce dose when the procedure supports them:

1. Time reduction: Practice the job outside the radiation field, pre-stage tools, and assign tasks so workers do not wait in the field.
2. Distance maximization: Use tooling, extension rods, or remote operation where compatible with the work and procedure.
3. Shielding: Use temporary shielding only when the source, material, handling, structural, and survey basis has been reviewed.

Radiation Work Permits

A Radiation Work Permit (RWP) or equivalent procedure record is a site-controlled document that states the conditions for radiation work. Requirements vary by license, Agreement State program, and facility procedure.

An RWP may include the work description, expected dose-rate rows, estimated worker dose prompts, local dose-budget prompt, required protective measures, dosimetry type, shielding or tooling, survey requirements, hold points, briefing requirements, and RSO or qualified reviewer signoff. A calculator report is not the RWP.

For industrial radiography under 10 CFR 34, daily job logs and operating procedures provide separate records that the RSO reviews as part of the radiation safety program.

RWPs are especially important for non-routine work where workers may not know the radiation environment. Use calculator outputs as supporting arithmetic only, then reconcile the result with the actual document, briefing, survey, dosimetry, access controls, and procedure.

Dose Tracking and Trending

Occupational dose monitoring is required by 10 CFR 20.1502 for workers who are likely to receive more than 10% of the applicable dose limit (500 mrem/yr TEDE for adults). Most industrial radiation workers wear thermoluminescent dosimeters (TLDs) or optically stimulated luminescence dosimeters (OSLDs) processed by an accredited (NVLAP) dosimetry service on a monthly or quarterly exchange cycle.

In addition to passive dosimeters, many workers carry alarming personal dosimeters (APDs, also called electronic dosimeters) that provide real-time dose and dose rate readings. APDs are not the legal record (the TLD/OSLD is), but they provide immediate feedback that helps workers manage their exposure during a job.

Dose trending means tracking each worker's dose over time, looking for patterns. A radiographer whose quarterly dose is consistently 200 mR for six quarters and then jumps to 600 mR in one quarter warrants investigation. Possible causes include a higher-activity source, more overtime, a change in technique, or a dosimetry error. The RSO should review the job logs, compare with coworkers doing similar work, and determine whether the increase reflects a real change in exposure conditions.

Facility dose reports should include: individual doses by monitoring period, cumulative annual dose, comparison to administrative limits, trend charts, and any investigation results for doses that exceeded action levels. NRC Regulatory Guide 8.7 provides guidance on occupational dose recordkeeping, and NRC Regulatory Guide 8.29 discusses risks from occupational radiation exposure.

Spreading Dose Across Workers and Shifts

One ALARA strategy is to distribute the dose among multiple workers rather than concentrating it in one person. If a job requires 4 hours in a 50 mR/hr field (200 mR total), sending two workers for 2 hours each gives each worker 100 mR instead of one worker getting the full 200 mR. This approach reduces individual dose but does not reduce collective dose (the sum of all individual doses, in person-rem).

Whether dose spreading is appropriate depends on the situation. If the job can be done efficiently by one skilled worker, adding a second worker who is less experienced may increase the total time (and collective dose) while reducing the individual dose. If the job is straightforward and can be easily divided, dose spreading is effective. The RSO must weigh individual dose reduction against collective dose and practicality.

For industrial radiography, the crew typically consists of two people (as required by 10 CFR 34.41 for all field radiography operations). Dose is naturally shared because both workers participate in setting up and breaking down the shot. If one worker's dose is trending higher, the RSO may rotate crew assignments so the higher-dose worker takes the assistant role (farther from the source during setup) on subsequent jobs.

Administrative dose limits set by the RSO below the 10 CFR 20 limits enable this management flexibility. If the administrative limit is 1 rem/quarter and a worker reaches 800 mR, the RSO can reassign that worker to lower-dose tasks for the remainder of the quarter while other workers complete the higher-dose work.

Emergency Dose Guidelines

Planned special exposures and emergency exposures are addressed separately from routine occupational limits. Under 10 CFR 20.1206, a planned special exposure allows a dose up to 5 rem TEDE in a single event (in addition to the annual limit), authorized by the licensee in advance under the specific conditions of 20.1206 (exceptional-situation justification, written authorization, informing the worker, and lifetime dose tracking). This provision is rarely used in industrial settings.

For emergency response involving a stuck source, overexposure, or lost source recovery, the NRC and EPA have published emergency dose guidelines. The general principle is that the dose accepted during an emergency should be commensurate with the benefit of the action. Saving a life may justify higher doses than recovering a source that is not posing an immediate public hazard.

The EPA Protective Action Guides (PAGs) and NCRP Report 116 provide dose guidelines for emergency responders. For perspective, a dose of 25 rem is the threshold for observable clinical effects (mild blood count changes). A dose of 100 rem can cause acute radiation syndrome. Industrial emergencies involving sealed sources rarely result in doses this high, but the RSO and emergency response team should know these thresholds and have a plan for dose management during a source recovery.

Key point: the best emergency dose management is prevention. Proper maintenance of exposure devices, adherence to operating procedures, and thorough pre-job planning prevent the events that lead to emergency exposures. The NRC's operating experience notifications and information notices document real incidents that provide lessons for preventing similar events.